netdev - Re: [PATCH stable<3.19] net: handle null iovec pointer in skb_copy_and_csum_datagram

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Date:	Fri, 23 Oct 2015 11:22:19 +0200
From:	Sabrina Dubroca <sd@...asysnail.net>
To:	Michal Kubecek <mkubecek@...e.cz>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	stable@...r.kernel.org, Herbert Xu <herbert@...dor.apana.org.au>,
	Sasha Levin <sasha.levin@...cle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, Zefan Li <lizefan@...wei.com>,
	Ben Hutchings <ben@...adent.org.uk>
Subject: Re: [PATCH stable<3.19] net: handle null iovec pointer in
 skb_copy_and_csum_datagram_iovec()

Hello Michal,

2015-10-23, 10:46:09 +0200, Michal Kubecek wrote:
> Mainline commit 89c22d8c3b27 ("net: Fix skb csum races when peeking")
> backport into pre-3.19 stable kernels introduces a regression causing
> null pointer dererefence in skb_copy_and_csum_datagram_iovec().
> 
> This commit only sets CHECKSUM_UNNECESSARY for non-shared skb, allowing
> udp_recvmsg() to take the "else" branch of if (skb_csum_unnecessary(skb))
> when called with null iovec (and len=0, e.g. when peeking for datagram
> size first). The problem is that unlike skb_copy_and_csum_datagram_msg()
> called in this path since 3.19, skb_copy_and_csum_datagram_iovec() does
> not handle null iov parameter and always dereferences iov->iov_len. This
> is especially harmful when udp_recvmsg() is called in kernel context,
> e.g. from kernel nfsd.
> 
> Band-aid skb_copy_and_csum_datagram_iovec() by testing iov for null and
> only checking the checksum in this case.
> 
> Signed-off-by: Michal Kubecek <mkubecek@...e.cz>
> ---

I ran into this problem too and that was my initial solution to this
problem as well, but actually, we need a more complete fix, like the
one I submitted a few days ago:

http://patchwork.ozlabs.org/patch/530642/

With your solution, userspace can still receive bogus EFAULT, or the
kernel ends up writing data to an unwanted memory location.


Thanks,

-- 
Sabrina
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